こんな感じになる
const Member = ( () => {
const map = new WeakMap
const priv = own => map.get( own ) || map.set( own, { __proto__: null } ).get( own )
return class Member {
constructor ( nameObj ) {
Object.assign( priv( this ), nameObj )
}
getName ( order = 'last-first' ) {
return order.split( '-' ).map( pos => priv( this )[ pos ] ).join( '-' )
}
}
} )( )

let mem = new Member( { first: '活男', middle: 'ジャイアント', last: '磯辺' } )
let name = mem.getName() // '磯辺-活男'
$( '#hoge' ).text( name )